The present invention relates to apparatus for the separation of a product from much smaller particles mixed therewith and more particularly to such apparatus in which the product and smaller particles are transported in a fluid stream. Such devices are often called "elutriators" in the trade.
The removal of fines (e.g., dust) and streamers from a pellet like product is often necessary in many industries. One such industry is plastics manufacturing in which pellets are employed as the raw material in the molding of plastic articles. The presence of fines and streamers mixed with the pellets hampers plastics processing in several ways. First of all, the presence of fines and streamers in the finished product is undesirable from a product standpoint. Second, the presence of fines and streamers in the molding machines themselves often results in production delays and inefficiencies because of machine down time required for cleaning between production runs.
Because of the long-standing nature of the problem of fines mixed with product, there have been numerous attempts to improve separation capability. Proposed systems for dealing with the problem include washing and settling apparatus and vertical air-assisted separators. One of the most effective separators known to date is disclosed by Avery in U.S. Pat. No. 4,159,941 the disclosure of which is incorporated herein by reference. This separator which is adapted for separating plastic pellets or granules from fines and streamers subjects the fluid borne material to three separating stages. In the first stage, the product and fines mixed therewith, travelling at a relatively high vertical velocity, are directed against an imperforate barrier or baffle. This baffle absorbs all of the linear momentum of the product pellets allowing them to drop under the influence of gravity to a product outlet. The fines, however, entrained in the air flow and having a much higher surface area to mass ratio than the product pellets, do not drop after impact but continue to be conveyed in the air stream. Not only does the impact of the product with the baffle allow the product to fall toward the outlet, but the inertial stresses which develop when the product is suddenly stopped by the baffle aid in dislodging any fines which have become stuck to the pellet. In the second stage of cleaning with the Avery apparatus, a reverse flowing stream of wash air enhances separation, and a third stage employing a chimney draft effect virtually eliminates the possibility of pellets being conveyed along with the fines out of the separator.
Although the separator disclosed by Avery is able consistently to remove a large proportion of the fines from the pellet product, e.g., 75 percent or more, the remaining fines still penalize the quality of the finished product. It is, therefore, an object of this invention to provide apparatus and method capable of separating a significantly higher fraction of fines and streamers from the product pellets than before economically and technically feasible.
A further object is such apparatus which is of relatively simple design, inexpensive to manufacture and maintain, and operable at relatively high flow rates.
Yet a further object of this invention is apparatus which can be rather simply retrofitted to existing separating equipment.
Still further objects and advantages of the invention disclosed herein will be in part specifically pointed out and will be in part apparent in what follows.